Satellite radio operators are providing digital radio broadcast services covering the entire continental United States. These services offer approximately 100 channels, of which nearly 50 channels in a typical configuration provides music with the remaining stations offering news, sports, talk and data channels. Digital radio may also be available in the near future from conventional analog radio broadcasters that will provide a terrestrial based system using signals co-located in the AM and FM bands.
The Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters broadcast the same program content in order to improve coverage reliability. Some mobile receivers as illustrated in FIGS. 1 and 2 are capable of simultaneously receiving signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation of multipath interference and addresses reception issues associated with blockage of the satellite signals.
In accordance with XM Satellite Radio, Inc.'s frequency plan, each of two geostationary satellites transmits identical or at least similar program content. The signals are transmitted with QPSK modulation from each satellite (hereinafter satellite 12 and satellite 14). For reliable reception, the LOS signals transmitted from satellite 12 are received, reformatted to Multi-Carrier Modulation (MCM) and rebroadcast by terrestrial repeaters 16. The assigned 12.5 MHZ bandwidth (hereinafter the “XM” band) is partitioned into two equal ensembles or program groups A and B. Each ensemble is transmitted by each satellite on a separate radio frequency (RF) carrier. Each RF carrier can support 50 channels or more of music, talk or data in Time Division Multiplex (TDM) format.
Existing SDARS systems (10) or mobile receiver units 18 use two antenna elements 15 and 17 that are typically co-located and omni directional, one for satellite reception, one for terrestrial reception. The satellite antenna is used to receive both satellite signals simultaneously. The terrestrial antenna is used to receive only the terrestrial signal.
As shown in FIG. 2, the channel decoder is divided into three stages where the first stage consists of a tuner 21 and digital demodulators 22, 25, and 28 that convert the RF signals to channel bit streams. The second stage includes TDM decoders 24, 27 and 29 that align the three signals and demultiplex the desired subchannels. The third stage performs forward error correction 32 and 36 on the channel bits and output the desired information bits. Additionally, such existing mobile receiver unit 18 can include elements 23 and 26 for signal quality detection to perform maximal ratio combining (30) as well elements 34 and 38 for signal quality detection to select (38) an error free signal among an forward error corrected satellite or terrestrial signal.
In existing SDAR radios, the two satellite signals are demodulated from the satellite antenna signal (from antenna 15) and the terrestrial signal is demodulated from the terrestrial antenna signal (from antenna 17). The three signals are aligned and demultiplexed independently by the TDM stage. Another way of describing this architecture is a two arm (antenna)/three branch (demodulator) radio as illustrated and discussed with regard to FIG. 2.
Although the existing two arm/three branch radio 18 of FIGS. 1 and 2 operates well, further improvements can be made to maximize signal reception and quality taking advantage of spatial diversity and signal combining techniques as will be further described below. Thus, in a digital audio radio system such as the system described above, a need exists for a system and technique for taking greater advantage of the signals being transmitted from satellite and terrestrial sources by using spatial diversity using multiple distributed antennas and signal combining techniques.